1. Field of the Invention
The present invention relates to a process for the purification of a gas stream comprising nitrogen monoxide NO and nitrogen oxides NOx by a purification unit with conversion of the nitrogen monoxide over a catalytic bed prior to said purification.
2. Related Art
Nitrogen oxides (NOx) are pollutants generally emitted during the combustion of fossil fuels. NOx compounds in the atmosphere create tropospheric ozone, which is toxic when it is inhaled. Furthermore, NOx compounds contribute to the formation of acid rain, which is harmful to plant and animal life, and property.
During the treatment and purification stages, in particular compression (followed by refrigeration), NOx compounds will generate acidic condensates, in particular: nitric acid, nitrous acid, and the like.
It is thus preferable to employ a process for scrubbing the flue gases which makes it possible to remove a portion of these impurities upstream of other stages of purification of the flue gases.
Stopping the nitrogen oxides can be carried out by using:                a low pressure washing column, with or without chemical additive, or        a column for washing with water, preferably under pressure, or        cryogenic separation, by distilling CO2-rich combustion flue gases comprising nitrogen oxides (NOx), where nitrogen oxides more highly oxidized than NO are separated from the flue gases.        
In particular, two types of processes are available for the treatment of NOx compounds: selective noncatalytic reduction (SNCR) and selective catalytic reduction (SCR). Both use ammonia (or a derivative, such as urea) to reduce the nitrogen oxides to molecular nitrogen, the difference between these two processes being the catalyst: it reduces the operating temperature and the consumption of reactant, it increases the capital costs but it also substantially increases the efficiency.
The SNCR process is characterized by:                high operating temperatures: between 850 and 1050° C. (according to whether or not additives are used and the starting NH/NO ratio);        a theoretical maximum degree of denitrification of 90% but, in practice, an efficiency of between 40 and 65% is observed;        a possible increase in the emissions of nitrous oxide, of CO and of ammonia (due to the use of ammonia as reactant).        
The SCR process is characterized by:                the use of ammonia or urea mixed with air. The gas laden with NH3 subsequently passes through a catalyst comprising several beds in a range of temperatures of between 250 and 380° C. The catalysts most often used are metal oxides on a TiO2 or Al2O3 support.        a high degree of reduction, often greater than 90%,        an operating temperature which increases in proportion as the starting amount of SO2 in the gas to be treated increases (between 170° C. and 540° C.),        fairly high capital costs.        
However, in some of the processes of the state of the art are the nitrogen oxides in the form of nitrogen monoxide NO stopped and they can thus, throughout the process, continue their slow conversion into a more oxidized form, causing problems in the downstream process. This is because NOx compounds can result, in a medium highly concentrated in water (indeed even saturated in water), as in the case of a gas produced by combustion, in the generation of strong acids (essentially nitric acid) capable of bringing about major problems of behavior of the materials used (for example corrosion). This is all the more marked when the conditions of the gas (temperature and pressure) are modified, for example when this gas is compressed for the purpose of subsequent use (additional purification by adsorption, for example, and the like).
Starting from here, a problem which is posed is that of providing an improved process for the purification of a gas stream comprising nitrogen monoxide NO and nitrogen oxides NOx with x>1.